Porcine class I major histocompatibility genes have been introduced into the genome of C57BL/1O mice. To date, four transgenic animals containing distinct class I genes have been generated, and demonstrated to transmit the transgene to progeny. One transgenic line expresses SLA antigen on its cell surface and rejects skin of normal C57BL/1O mice, indicating that the foreign SLA antigen is recognized as a functional transplantation antigen. Expression of this SLA gene, PD1, parallels that observed in the swine, indicating that its expression is regulated. In vivo treatment of the transgenic with alpha, beta-interferon increases PD1 expression in a number of tissues. This increase parallels that observed for the endogenous transplantation antigen, Kb, but differs markedly from the differentiation antigen, Qa-2.